It has been known for decades that hydraulic fracture stimulation can improve the productivity of a well in a tight petroleum or gas reservoir, because a long conductive fracture transforms the flow path that petroleum or natural gas must take to enter the wellbore. It is also known that the fractures created close back shut over time and the well stops producing. How long a well produces depends on the composition of the oil bearing formation and the type of fracture fluid used. Oil wells which become non-producing can often be re-fractured and become producing wells again. As much as 70 percent of the oil is often still in the ground when a well becomes non-producing.
It is common practice to stimulate recovery of fluids from subterranean porous formations by fracturing the porous formation to open new pathways for flow to the wellbore. One commonly used technique for fracturing formations is hydrofracturing. In such fracturing operations, a fracturing fluid (or “frac fluid”) is hydraulically injected into a wellbore penetrating the subterranean formation and is forced against the formation strata by very high pressure. The formation strata or rock is forced to crack and fracture, and a proppant is placed in the fracture by movement of a viscous-fluid containing proppant into the crack in the rock. The resulting fracture, with proppant in place, provides improved flow of the recoverable fluid, i.e., oil, gas or water, into the wellbore.
Fracturing fluids customarily comprise a thickened or gelled aqueous solution which has suspended therein “proppant” particles that are substantially insoluble in the fluids of the formation. Proppant particles carried by the fracturing fluid remain in the fracture created, thus propping open the fracture when the fracturing pressure is released and the well is put into production. Suitable proppant materials include sand, walnut shells, sintered bauxite, or similar materials. The “propped” fracture provides a larger flow channel to the wellbore through which an increased quantity of hydrocarbons can flow, thereby increasing the production rate of a well.
Dispersing fracture fluids are those which include aqueous solutions of monovalent cation salts, such as potassium chloride (KCl) and sodium chloride (NaCl), ammonium chloride (NH4Cl) and other salts. For example, dispersing fracture fluids may include alkoxylated fatty amines and an alkoxylated quaternary ammonium salt organic sulfates, phosphates, chlorides, fluorides, citrates, acetates, tartrates, hydrogenphosphates or a mixture thereof. Other examples include ammonium sulfate, sodium sulfate, magnesium sulfate, aluminum sulfate, ammonium hydrogen phosphate, sodium hydrogen phosphate, and potassium hydrogen phosphate.
Aggregating fracture fluids are those which include aqueous solutions of di and trivalent cation salts, including calcium chloride (CaCl2), ferric chloride (FeCl3), magnesium chloride (MgCl2), and other salts, for example, such as di, and trivalent metal salts of carboxylic acids. Exemplary salts can also include ferric oxalate, ferric ammonium citrate, barium acetate, aluminum lactate, and magnesium formate.
A dispersing fracture solution in the fracture zone will disperse clays and other earthen particles and allow them to be carried by the flow-back fluids out of the hydrocarbon producing fracture zone. This process increases hydrocarbon production when the pay zone is not mostly clay.
In contrast, an aggregating fracture solution, such as CaCl2, will aggregate and bind clays and other earthen materials. This stabilizes the fracture zone but will eventually clog and occlude the pay zone with the clay particles that are not aggregated by the CaCl2.
The fracturing operation is intended to create fractures that extend from the wellbore into the target oil or gas formations. Injected fluids have been known to travel as far as 3,000 feet from the well. Although attempts are made to design fracturing jobs to create an optimum network of fractures in an oil or gas formation, fracture growth is often extremely complex, unpredictable and uncontrollable. Computer models are used to simulate fracture pathways, but the few experiments in which fractures have been exposed through coring or mining have shown that hydraulic fractures can behave much differently than predicted by models.
Many fracturing fluid materials, therefore, when used in large concentrations, have relatively poor “clean-up” properties, meaning that such fluids undesirably reduce the permeability of the formation and proppant pack after fracturing the formation. Detailed studies of polysaccharide recovery in the field after hydraulic fracturing operations indicate that more than sixty percent of the total mass of polysaccharide pumped during the treatment maybe left in the fracture at the time gas or oil begins to be produced in commercial quantities.
In general, a single fracturing operation in a shallow gas well (such as a coalbed methane well) may use several hundreds of thousands of gallons of water. Slickwater fracs, which are commonly used in shale gas formations, have been known to use up to five million gallons of water to fracture on one horizontal well.
In most cases, fresh water is used to fracture wells because it is more effective than using wastewater from other wells. If wastewater is used, the water must be heavily treated with chemicals to kill bacteria that cause corrosion, scaling and other problems. Even freshwater fracturing operations, however, contain numerous chemicals such as biocides, acids, scale inhibitors, friction reducers, surfactants and others, but the names and volumes of the chemicals used on a specific fracturing job are almost never fully disclosed. In general, it is known that many fracturing fluid chemicals are toxic to human and wildlife, and some are known to cause cancer or are endocrine disruptors.
There continues to exist in the well drilling and hydraulic fracturing arts, a need for a fracture fluid that is effective in a variety of soil conditions, and has properties of both an aggregating fluid and a dispersing fluid, but is also non-toxic and not a source of hazardous waste.